1. Field of the Invention
The present invention relates to a multi-beam scanning device used in a laser writing optical system of an image formation apparatus, such as a color laser printer, a digital copier, or the like.
2. Description of the Related Art
In a multi-beam scanning device which uses two semiconductor laser arrays as light sources, a method for adjusting the sub-scanning beam pitch on a to-be-scanned body such as a photoconductor drum, is proposed by Japanese laid-open patent application No. 2000-75227, as an example of adjusting the sub-scanning beam pitch on a to-be-scanned body such as a photoconductor drum.
According to the above-mentioned publication, a light source unit is provided which includes a holding member which holds integrally two semiconductor laser arrays together with two coupling lenses, and, the sub-scanning beam pitch on a to-be-scanned body is adjusted as this light source unit is rotated about a rotation axis which corresponds to the optical axis of the optical system.
Moreover, according to the same publication, even in a case of a type in which beams emitted from the two semiconductor laser arrays are combined by utilizing polarization characteristic of the laser, the above-mentioned method can also be applied for precisely adjusting the sub-scanning pitch on the to-be-scanned surface.
However, according to the above-mentioned art, there is a possibility that the main scanning beam pitch is also changed due to the rotation of the light source unit, and, also, when the mounting precision of the scanning optical system or the mounting precision of the light source unit may not be sufficiently secured, the state of the beam arrangement before the adjustment is not satisfactory one, and, as a result, there is a possibility that the predetermined value on the sub-scanning beam pitch cannot be obtained only by the rotation of the light source unit.
Therefore, when an electrostatic latent image is formed on a photoconductor drum through the multi-beam scanning device having such a light source unit, and then, a toner image is obtained from the latent image through a well-known development and fixing processes, the thus-obtained final image may be degraded such as fluctuation on vertical lines or the like, due to the change in the main scanning beam pitch after the sub-scanning beam pitch adjustment and/or insufficient state of beam arrangement before the sub-scanning beam pitch adjustment.
The present invention has been devised in order to solve these problems, and an object of the present invention is to provide a multi-beam scanning device by which a satisfactory final image can be obtained in an image formation apparatus employing this scanning device even after the sub-scanning beam pitch adjustment has been made.
A multi-beam scanning device according to the present invention, scanning a to-be-scanned surface with a plurality of laser beams simultaneously, comprises:
a light-source unit comprising a plurality of laser arrays, each comprising a plurality of light-emitting points, a corresponding plurality of coupling lenses coupling laser beams emitted from the plurality of laser arrays, and a holding member integrally holding the plurality of laser arrays and plurality of coupling lenses rotatably approximately about optical axes on the laser beams; and
a scanning optical system deflecting the laser beams emitted from the light-source unit and imaging them onto the to-be-scanned surface.
There, the light-source unit and scanning optical system are configured so that the following equation be satisfied:
AY=|qxc3x97cos xcfx86xc3x97mYxc3x97(nxe2x88x921)/(2xc3x97fcolxc3x97tan xcex8xc3x97cos xcex3xc3x97mZ)|xe2x89xa60.1 
where:
n denotes the number of light-emitting points on each LD array;
q denotes an interval between each adjacent ones of the light-emitting points;
xcfx86 denotes an inclination angle of each laser array with respect to a sub-scanning direction;
mY denotes a magnification of the scanning optical system on main scanning direction;
mZ denotes a magnification of the scanning optical system on sub-scanning direction;
fcol denotes the focal length of each coupling lens;
xcex8 denotes half a crossing angle at which the laser beams emitted from the plurality of laser arrays cross therebetween;
xcex3 denotes a maximum required rotational angle of the light-source unit in case of adjustment.
Further or alternatively, in the multi-beam scanning device, the light-source unit and scanning optical system are configured so that the following equation be satisfied:
AZ=|qxc3x97sine xcfx86xc3x97(nxe2x88x921)/(2xc3x97fcolxc3x97tan xcex8xc3x97cos xcex3)|xe2x89xa60.1 
Thereby, even in case, an error in scanning line interval occurring due to optical-axis manufacture/assembling error or so between the plurality of laser arrays should be corrected by rotating (xcex3 rotation) the holding unit in an adjustment work, a newly occurring scanning line interval error along the sub-scanning direction and/or beam spot interval error along the main scanning direction due to the above-mentioned adjustment work can be controlled to be made within a permissible range.
A multi-beam scanning device according to another aspect of the present invention, scanning a to-be-scanned surface with a plurality of laser beams simultaneously, comprises:
a light-source unit comprising a plurality of laser arrays, each comprising a plurality of light-emitting points, a corresponding plurality of coupling lenses coupling laser beams emitted from the plurality of laser arrays, and a holding member integrally holding the plurality of laser arrays and plurality of coupling lenses rotatably approximately about optical axes on the laser beams;
a scanning optical system deflecting the laser beams emitted from the light-source unit and imaging them onto the to-be-scanned surface; and
a part switching a scanning density on the to-be-scanned surface by rotating the light-source unit approximately about the optical axes on the laser beams emitted therefrom.
Thereby, even with a simple arrangement, it is possible to easily perform switching of the scanning density on the to-be-scanned surface in the multi-beam scanning device employing the plurality of laser arrays by appropriately rotating (xcex3 rotation) the holding member integrally holding these laser arrays.
Further, it is preferably that the light-source unit and scanning optical system are configured so that the following formula be satisfied:
xcex94RY=|{(nxe2x88x921)xc3x97(2nxe2x88x921)/2}xc3x97{(qxc3x97cos xcfx86xc3x97mYxc3x97d)/(fcolxc3x97tan xcex8xc3x97mZ)}|xe2x89xa6d/4 
where:
d denotes scanning line interval;
n denotes the number of light-emitting points on each laser array;
q denotes an interval between each adjacent ones of the light-emitting points;
xcfx86 denotes an inclination angle of each laser array with respect to a sub-scanning direction;
mY denotes a magnification of the scanning optical system on main scanning direction;
mZ denotes a magnification of the scanning optical system oh sub-scanning direction;
fcol denotes the focal length of each coupling lens;
xcex8 denotes half a crossing angle at which the laser beams emitted from the plurality of laser arrays cross therebetween;
xcex94RY denotes the main-scanning-directional component of beam-spot interval between both ends of beam spots on the to-be-scanned surface from each laser array.
Further or alternatively, it is preferable that the light-source unit and scanning optical system are configured so that the following formula be satisfied:
xe2x80x83xcex94RZ=|{(nxe2x88x921)xc3x97(2nxe2x88x921)/2}xc3x97{(qxc3x97sin xcfx86xc3x97d)/(fcolxc3x97tan xcex8)}|xe2x89xa6d/4
where xcex94RZ denotes the sub-scanning-directional component of beam-spot interval between both ends of beam spots on the to-be-scanned surface from each laser array.
Thereby, it is possible to control within a predetermined range the beam spot interval error on the to-be-scanned surface occurring due to the xcex3 rotation of the holding member for the switching of the scanning density on the to-be-scanned surface.